The present invention relates to a semi-batch process for the production of low molecular weight polyoxyalkylene polyols. This semi-batch process oxyalkylates one or more starters in the presence of a DMC catalyst in which the alkylene oxide feed is completed with a space time yield of greater than or equal to 250 kg/m3/hr.
The production of low molecular weight products is important as these can be used directly to replace products made using conventional KOH technology or these can be used as starters to make long chain IMPACT (DMC) or KOH products. One of the challenges of making low molecular weight products with DMC catalysts is deactivation of the catalyst. It is well known that deactivation of the DMC catalyst can be resolved by increasing the catalyst concentration. This is not desirable, however, due to the expense of the catalyst. It is preferred to maintain a constant catalyst level of 60 ppm or lower. It is also preferred to maintain the reaction temperature at 130° C. or lower to minimize potential negative effects of reaction temperature such as color formation.
One advantage of double metal cyanide catalysts is that they do not promote the rearrangement of propylene oxide into propenyl alcohol which acts as a monofunctional initiator in propylene oxide polymerization. The presence of propenyl alcohol promotes the formation of monoalcohols which are an impurity in the process.
Another advantage of double metal cyanide catalysts includes the ability to leave the catalyst residue in the product. This results in lower production cost since the catalyst residues do not have to be removed from the polyoxyalkylene polyol prior to use. This is also another reason that it is desirable to minimize the amount of catalyst used.
While double metal cyanide catalysts provide numerous advantages in preparing polyoxyalkylene polyols, there are, unfortunately, some disadvantages to this type of catalysis. These disadvantages include the tendency of the catalyst to deactivate in the presence of high concentrations of hydroxyl groups, the inability to polymerize in the presence of low molecular weight initiators such as glycerin, and the fact that, in addition to the desired product, DMC catalysts produce a small quantity of a very high molecular weight (i.e. at least 100,000 MW and higher) polymer. This high molecular weight polymer is commonly referred to as high molecular weight tail. High molecular weight tail causes difficulties with the foaming process when reacting a polyol with a polyisocyanate to produce a polyurethane foam.
There have been numerous efforts over the years to improve and extend double metal cyanide catalysis to enable effective oxyalkylation of low molecular weight starters such as glycerin, and to produce low molecular weight polyoxyalkylene polyols. The acidification of starters is one method found to be effective. Another method is to use starters with ultra-low water content.
Low molecular weight or high hydroxyl number polyoxyalkylene polyols are characterized by a high percentage of the starter used to make the final product. As an example, a 400 MW, glycerin based polyoxyalkylene polyol contains 23% by weight of glycerin and 77% by weight of alkylene oxide (weight percent based on the total product); whereas a 3000 MW, glycerin based polyoxyalkylene polyol contains 3% by weight of glycerin and 97% by weight of alkylene oxide (weight percent based on the total product). The 400 MW product requires an overall starter to alkylene oxide ratio of 0.3 while a 3000 MW product requires an overall starter to alkylene oxide ratio of 0.03. As used herein, the starter is the total weight of starter required and the alkylene oxide is the total weight of alkylene oxide required. The higher ratio of starter to alkylene oxide or higher concentration of starter required for the low molecular weight polyoxyalkylene polyols presents challenges in a DMC catalyzed process because of the tendency of the starters to inhibit the activity or deactivate the DMC catalyst. This reduction in activity or deactivation of the DMC catalyst is measured by a pressure increase in the semi-batch reaction caused by elevated free oxide concentration. Pressure increases are often observed after the low molecular starter is introduced to the reactor of a semi-batch process using a Continuous Addition of Starter (CAOS) approach to make low molecular weight polyoxyalklyene polyols. Typically, in such an approach the DMC catalyst is first activated in the presence of an initial starter or heel and reacted with alkylene oxides before the continuous starter is introduced. This is, known as a Pre-CAOS build ratio. The initial pressure increase observed after the introduction of the CAOS feed demonstrates that the DMC catalyst is not fully activated or becomes partially inhibited by the starter introduction. The Pre-CAOS build ratio is part of the process that allows the catalyst to become activated, and thus prevents the batch from completely deactivating. As the reach of DMC technology is extended to products having lower and lower molecular weights, one of the most significant hurdles to be overcome is the tendency of the system to deactivate via elevated free oxide content soon after a high instantaneous starter/alkylene oxide ratio feed commences, and before the catalyst has become fully active. Although extending the Pre-CAOS build ratio will help prevent this, it then becomes necessary to compensate for the delay by feeding at an even higher instantaneous starter/alkylene oxide ratio once the CAOS feed commences as the denominator has decreased. The instantaneous ratio is defined by the total continuous starter weight over the alkylene oxide weight fed during the continuous starter feed.
The second area where the pressure can increase quicker than usual demonstrating an increase in elevated free oxide concentration and thus a reduction in the catalyst activity is at the end of the CAOS feed. This indicates a gradual loss of catalyst activity during the main portion of the oxyalkoxylation as the catalyst concentration in the reaction media decreases.
Thus, a need exists for a method to accelerate the rate at which the DMC catalyst becomes active and a method to maintain catalyst activity. It has recently been discovered that a partial EO co-feed and/or increase in feed rates of alkylene oxides appear to do just that. The fact that EO and/or increased feed rates can accelerate DMC catalyst activation may lead to the development of processes for low molecular weight polyols that do not require special raw material handling (such as ultra-low water levels in the CAOS feeds) which can be difficult to meet on a commercial scale.